Resistance element



Oct. 23, 1951 c. H. sPARKLxN 2,572,069

RESISTANCE ELEMENT Original Filed Jan. 15, 1947 2 SHEETS- SHEET l Oct. 23, 1951 c. H. SPARKLIN 2,572,069

RESISTANCE ELEMENT Original Filed Jan. 15, 1947 2 SHEETS-SHEET 2 Patented Oct. 23, 1951 n itEsis'ra'NoE ELEMENT Charles H. Sparklin,` Chicago, Ill., assignorto Birtman Electric Company, a corporation of Illinois Original application January 15, 1947, Serial No.

722,127, now Patent No. 2,512,692, dated'June "27, 1950. Dividedand this 4, 1949, Serial No. 108,614

s claires( (c1. 201-63) This invention relates to a resistance element such as may be usedin an electric iron sole plate or the like.

This application is a division of my copending application Serial No. 722,127, filed January ,15, 1947, now Patent 2,512,692, issued June 271950, which in turn was a continuation-in-part of my application Serial No. 524,850, filed March 3, 1944, now abandoned.

The resistance element of this invention is of extremely sturdy construction that exhibitsV extremely long service life when used in electric iron sole plates or the like.' Where most resistance elements operate from 50 to 750 hours before failure, while the best elements sometimes have a life as long as 5,000 hours, an electric iron containing in the sole plate the new resistance element described and claimed herein has been operated for 20,000 hours without failure, at which point the test was stopped.

The resistance element will be described in conjunction with an electric ironsole'plate containing the same. It is, of course, true that the resistance element will be used in other installations and can be used not only as a heating element but also as a component in an electric circuit to supply resistance to the flow of electricity.

The invention is illustrated in the drawings in which Fig. l is a plan view of the sole plate of a preferred form of the iron; Fig. 2 is a section taken along the line 2 2 of Fig. 1; Fig. 3 is' a sectional view like Fig. 2 illustrating the first phase of forming the embedded resistance element; Fig. 4 is a similar view illustrating the second step; Fig. 5 is a sectional elevation of a 2; Fig. 6 is a view similar to Fig. 4, illustrating the iron after the completion of the third and fourth steps; Fig. 7 is a view of one form of the resistance element; Fig. 8 is a view similar to Fig. 6 and showing the iron after completion of the fifth forming step; Fig. 9 is a sectionalelevation illustrating a plunger and stamping employed in the fifth forming step; Fig. 10 is a view illustrating a preferred form of resistance element mounting; Fig. 11 is a detail view of the end of the ceramic device shown in Fig. 10 illustrating the method of introducing the resistance wire into the ceramic core; Fig. 12 is a plan view of the iron with the resistance element installed and attached; and Fig.. 13` is a sectional view taken along the line 'I3- I3 in Fig. 10.

The sole plate shown Ain the drawings and including the new resistance element comprises a metal platefzll of any suitable met'al which'may suitable plunger employed in carrying out stepv application August be, for example, aluminum or steel, upon the upper surface of which isa pair of converging slots 2| and 22 bounded by the integral anges 23, 24, 25 and 26, which may be formed with the sole plate by a single casting operation.

when pressed. The volume of material employed will depend upon the compressibility of the powder and the thickness of refractory and insulating coating desired. Ordinarily the amount used should be such'as to form a thin semitubular coating having a thickness of from 11; to 1A, inch. Ordinarilyabout l inch depth of magnesium oxide powder will produce a coating of suitable thickness upon compression. A layer of this powder 30 is indicated in Fig. 3. The powder is then compressed, for example, by a plunger 3l having a semi-cylindrical head 32 under a pressure of from 2.5 to 6 tons per square inch, for example, 3 tons per square inch. The neck 33 of the plunger extends the complete length of one of the slots 2| or 22 and fits tightly enough therein so ,that the powder will not be forced out around it. The head 32 is narrower than the neck and forms two longitudinal shoulders 34 and 35 which correspond to the thickness of the .coating 36 which is produced upon com pression of the powder 30. The operation of the plunger 3l upon the powder in the slots produces a coherent self-sustaining semi-tubular bed of refractory insulating material, as shown in Fig. 4. The resistance Yelement (Fig. 7) comprising the resist-ance memberll in the form of a wire arranged in a helix having closely spaced convolutions and preferably formed about a ceramic core or rod M, is then laid within the kbed formed by the coating 3,6.,v The resistance member4 or wire may be of the usual Nichrome wire. The method lof embeddingl which is here employed permits the use of larger wire thanv is usually employed. A suitable wire has adiameter of 0.028 inch and it may be coiled around a ceramic core having a diameter about lluinch inconvolutions separated by as little as .020 inch.y The ends of the wire, as they* extend beyond the ceramic core,`ar,e preferably doubled back as indicated at 42.." This-formsa loop43 at each end of the Wire which may be used for attachment to the posts l and 55, and also has been found desirable in protecting the wire. The life of the resistance element is considerably greater when the ends of the wire have been doubled back as indicated.

When the resistance element has been inserted upon the bed 36, additional powder 5U is added substantially to ll the slots 2| and 22, as shown in Fig. 6. This powder is suitably the same as the powder employed for the lunderlying bed. The powder is then compressed under a considerably greater pressure such as 25 to 40 tons per square inch, for example 38 tons per square inch, as shown in Figs. 8 and 9. Thislast compressing is preferably by the use of aplungerl having a neck 52 which presses upon a pressure stamping 53. rhis stamping is in the form of a boat having the same length as This boat has a bottom 54, and integral upstanding side walls 55 and 5.6 which are-preferably upturned at an angle slightly less than 90. The metal of the boat is selected to have considerable Vresilience so that the side walls 55 and 55 tend to engage the flanges 23 and 24 which form the Walls of the slot, and to retain the boat therein.

-The boat thus acts as a closuremember tightll7 sealing the insulating refractory material within the sole plate. It also acts as a heat distributing member which carries heat radiated or conducted into it from the resistance element into the metal of the sole plate and thus considerably reduces radiation losses to the air.

The bottom layer bed 36 having been compressed under the relatively light pressure of 2.5 to 6 tons per square inch is still deformable under additional pressure. When the resistance element wire 55 and core Gl unit is laid on the bed, covered with powdered insulating materialA 50, and the powder 50 compressed at a pressure of 25 toll0 tons persquare inch, the wire lll and core 4l is pressed into the bed 36 so that the wire penetrates the bed. The tremendous final pressure makes a unitary rigid insulating structure consisting of the core 4| and the surrounding insulating material with the resistance element l0 rigidly held in the surrounding material. The insulating structure is quite solid and completely fills the spaces between the inner surfaces of the slots 2l and 22 and the stampings 53 so that no air pockets are left to later expand under heat. The resistance element wire is solidly held on all sides so that when heated and cooled it can not move under the forces of expansion and contraction. The insulating material and resistance element wire is, in effect, formed into a unitary solid structure with the sole plate and the resistance element lasts as long as the sole plate itself.

During the formation of the embedded resistyance element, the ends of the slots may be closed in any desirable fashion, either by temporary closure members applied individually or collectively to the slots, or by mechanism which may be a permanent part of the pressing apparatus. This closing off of the ends of the slots is facilitated by having the ends parallel and by having the slots of equal length. Thus a single closing olf element may be employed at each end if desired.

An electric iron containing the new sole plate has been operated for 20,000 hours without failure, at which point the test was stopped. The ordinary electric iron heating unit operates from 50 to'750 hours before failure, while the best the slots 2| and'22.

previous irons with a heating element insulated with solid sheet mica sometimes had a life as long as 5,000 hours. The present structure will last indefinitely even when operated at as much as 2,000 watts. It has beenfound that where a cast iron sole plate is used with the element, the element may be operated at such a temperature that the cast iron will clinker, and the element will continue to operate.

In Figs. l0 and 1l there is shown a preferred form of resistance element. This comprises a ceramic or other insulating material of considerable strength in theform of an elongated generally cylindrical member 60, at each end of which is'a cut-away portion 5| extending from the end of the'last helical convolution 52 to the end of,l the core." The material is cut away on substantially a diameter 63 as shown in Fig. 13, and a'feeding hole 54 isformed transversely of the neck 65 remaining. In winding the resistance wire upon the core, the twisted double end 'l0 of the resistance wire is passed inwardly through the opening 64, as shown in Fig. l1, and is then bent at'right angles thereto. This effectively anchors the wire and permits the wire to be wound tightly upon the grooves 52. On the completion of the winding, the residual end of the wire, likewise doubled back upon itself and twisted, is drawn inwardly through the hole B4 and bent axially of the rod.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

I claim:

1. A resistance element comprising a refractory insulating rod, the rod being cut away at each end on one side of the axis of the rod with a transverse opening being provided at each end through the portion of the rod remaining from the cut away part, and a continuous relatively flexible resistance wire extending outwardly through the transverse opening at one end of the rod and thence bent to pass in helical convolutions around the rod to the opposite end thereof and bent to extend inwardly through the transverse opening at said opposite end, the wire being held tightly in said helical convolutions by its own tensional deformation in said transverse openings, each end of said wire being normally positioned in at least a portion of a cut away part 4at a corresponding end of the rod.

2. The resistance element of claim l wherein each end of the wire is doubled back and twisted upon itself to extend back through the corresponding opening.

3. The resistance element of claim wherein said rod is provided with a helical groove in which said helical convolutions are located, and each end of the wire is doubled back and twisted upon itself to extend back through the corresponding opening.

CHARLES H. SPARKLIN.

REFERENCES CITED rIhe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,642,223 Baker Sept. 13, 1927 FOREIGN PATENTS Number Country Date 24,986 Great Britain Nov. 14, 1902 

